Method for making light emitting diode

ABSTRACT

A method for making light emitting diode, the method includes the following steps. First, a substrate having an epitaxial growth surface is provided. Second, a carbon nanotube layer is suspended above the epitaxial growth surface. Third, a first semiconductor layer, an active layer and a second semiconductor layer are grown on the epitaxial growth surface in that order, wherein the first semiconductor layer includes a buffer layer, an intrinsic semiconductor layer, and a doped semiconductor layer stacked in that order. Fourth, the doped semiconductor layer is exposed by removing the substrate, the buffer layer, and the intrinsic semiconductor layer. Fifth, a first electrode is prepared on the first semiconductor layer and a second electrode is prepared on the second semiconductor layer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.13/288,213, filed on Nov. 3, 2011, entitled, “METHOD FOR MAKING LIGHTEMITTING DIODE,” which claims all benefits accruing under 35 U.S.C. §119from China Patent Application No. 201110110728.6, filed on Apr. 29, 2011in the China Intellectual Property Office. The disclosures of theabove-identified applications are incorporated herein by reference.

BACKGROUND

1. Technical Field

The present disclosure relates to a light emitting diode (LED) andmethod for making the same.

2. Description of Related Art

In recent years, highly efficient LEDs made with GaN-basedsemiconductors have become widely used in different technologies, suchas in display devices, large electronic bill boards, street lights, carlights, and other illumination applications. LEDs are environmentallyfriendly, long working life, and low power consumption.

A conventional LED commonly includes an N-type semiconductor layer, aP-type semiconductor layer, an active layer, an N-type electrode, and aP-type electrode. The active layer is located between the N-typesemiconductor layer and the P-type semiconductor layer. The P-typeelectrode is located on the P-type semiconductor layer. The N-typeelectrode is located on the N-type semiconductor layer. Typically, theP-type electrode is transparent. In operation, a positive voltage and anegative voltage are applied respectively to the P-type semiconductorlayer and the N-type semiconductor layer. Thus, cavities in the P-typesemiconductor layer and electrons in the N-type semiconductor layer canenter the active layer and combine with each other to emit visiblelight.

However, extraction efficiency of LEDs is low for the following reasons:first, the typical semiconductor materials have a higher refractionindex than that of air, therefore, large-angle lights emitted from theactive layer may be internally reflected in LEDs, so that a largeportion of the lights emitted from the active layer will remain in theLEDs; second, the current is limited under the P-type electrode, so thatthe conduction of the current along a direction away from the P-typeelectrode is weaken, thus the lights emitted from the active layer isreduced. The extraction efficiency of LEDs is low so that the heatproduced in the LEDs is remained in the inner of the LED. Therefore, theproperty of the semiconductor materials is damaged and the life span ofthe LED is shorter. As a result, the large-scale application of the LEDsis affected.

What is needed, therefore, is a LED and a method for making the same,which can overcome the above-described shortcomings

BRIEF DESCRIPTION OF THE DRAWINGS

Many aspects of the embodiments can be better understood with referenceto the following drawings. The components in the drawings are notnecessarily drawn to scale, the emphasis instead being placed uponclearly illustrating the principles of the embodiments. Moreover, in thedrawings, like reference numerals designate corresponding partsthroughout the several views.

FIG. 1 is a flowchart of one embodiment of a method for making an LED.

FIG. 2 is a Scanning Electron Microscope (SEM) image of a drawn carbonnanotube film used in the method of FIG. 1.

FIG. 3 is a schematic structural view of a carbon nanotube segment ofthe drawn carbon nanotube film of FIG. 2.

FIG. 4 is an SEM image of cross-stacked drawn carbon nanotube films usedin the method of FIG. 1.

FIG. 5 is an SEM image of untwisted carbon nanotube wires used in themethod of FIG. 1.

FIG. 6 is an SEM image of twisted carbon nanotube wires used in themethod of FIG. 1.

FIG. 7 is a schematic structural view of an LED making by the method inFIG. 1

FIG. 8 is a schematic structural view of one embodiment of an LED.

FIG. 9 is a flowchart of another embodiment of a method for making anLED.

FIG. 10 is a schematic structural view of an LED made by the method inFIG. 9.

FIG. 11 is a flowchart of another embodiment of a method for making anLED.

FIG. 12 is a schematic structural view of an LED made by the method inFIG. 11.

FIG. 13 is a schematic structural view of one embodiment of an LED.

FIG. 14 is a magnified schematic strutural view of the firstsemiconductor layer 120 of the LED in FIG. 1.

DETAILED DESCRIPTION

The disclosure is illustrated by way of example and not by way oflimitation in the figures of the accompanying drawings in which likereferences indicate similar elements. It should be noted that referencesto “an” or “one” embodiment in this disclosure are not necessarily tothe same embodiment, and such references mean at least one.

Referring to FIG. 1, a method for making an LED of one embodimentincludes the following steps:

S11, providing a substrate 100 having an epitaxial growth surface 101;

S12, suspending a carbon nanotube layer 102 above the epitaxial growthsurface 101; and

S13, growing a first semiconductor layer 120, an active layer 130, and asecond semiconductor layer 140 on the epitaxial growth surface 101 inthat order, in which the carbon nanotube layer 102 is enclosed in thefirst semiconductor layer 120;

S14, etching a portion of the second semiconductor layer 140 and theactive layer 130 to expose a portion of the first semiconductor layer120; and

S15, preparing a first electrode 150 on the first semiconductor layer120 and preparing a second electrode 160 on the second semiconductorlayer 140.

In step S11, the epitaxial growth surface 101 can be used to grow thefirst semiconductor layer 120. The epitaxial growth surface 101 is aclean and smooth surface. The substrate 100 can be made of transparentmaterial. The substrate 100 is used to support the first semiconductorlayer 120. The substrate 100 can be a single-layer structure or amulti-layered structure. If the substrate 100 is a single-layerstructure, the substrate 100 can be a single crystal structure having acrystal face. The crystal face can be used as the epitaxial growthsurface 101. If the substrate 100 is the single-crystal structure, thematerial of the substrate 100 can be made of SOI (silicon on insulator),LiGaO₂, LiAlO₂, Al₂O₃, Si, GaAs, GaN, GaSb, InN, InP, InAs, InSb, AlP,AlAs, AlSb, AlN, SiC, SiGe, GaMnAs, GaAlAs, GaInAs, GaAlN, GaInN, AlInN,GaAsP, InGaN, AlGaInN, or AlGaInP. If the substrate 100 is a multi-layerstructure, the substrate 100 should include at least one layer of theabove-described single crystal structure having a crystal face. Thematerial of the substrate 100 can be selected according to the materialof the first semiconductor layer 120 which will be grown on thesubstrate 100 in step S30. The size, thickness, and shape of thesubstrate 100 can be selected according to need. In one embodiment, thesubstrate 100 is made of sapphire.

In step S12, the carbon nanotube layer 102 includes a number of carbonnanotubes. A thickness of the carbon nanotube layer 102 is in a rangefrom 1 nm to 100 μm, for example the thickness can be about 1 nm, 10 nm,200 nm, 1 μm, or 10 μm. In one embodiment, the thickness of the carbonnanotube layer 102 is about 100 nm. The length and diameter of thecarbon nanotubes in the carbon nanotube layer 102 are selected accordingto need. The carbon nanotubes in the carbon nanotube layer 102 can besingle-walled, double-walled, multi-walled carbon nanotubes, orcombinations thereof.

The carbon nanotube layer 102 forms a pattern so part of the epitaxialgrowth surface 101 can be exposed from the patterned carbon nanotubelayer 102 after the carbon nanotube layer 102 is placed on the epitaxialgrowth surface 101. Thus, the first semiconductor layer 120 can growfrom the exposed epitaxial growth surface 101.

The patterned carbon nanotube layer 102 defines a number of apertures105. The apertures 105 are dispersed uniformly. The apertures 105 extendthrough the carbon nanotube layer 102 along a thickness direction of thecarbon nanotube layer 102. Therefore, the carbon nanotube layer 102 is agraphical structure. The carbon nanotube layer 102 covers the epitaxialgrowth surface 101 of the substrate 100. A portion of the epitaxialgrowth surface 101 is then exposed from the apertures 105 of the carbonnanotube layer 102, and the first semiconductor layer 120 grows from theapertures 105 of the carbon nanotube layer 102 in step S13. Theapertures 105 can be a hole defined by several adjacent carbon nanotubesor a gap defined by two substantially parallel carbon nanotubes andextending along axial direction of the carbon nanotubes. The size of theapertures 105 can be the diameter of the hole or width of the gap, andcan be in a range from about 10 nm to about 500 μm. The hole-shapedapertures 105 and the gap shaped apertures 105 can exist in thepatterned carbon nanotube layer 102 at the same time. The sizes of theapertures 105 can be different. The sizes of the apertures 105 can be ina range from about 10 nm to about 300 μm, for example the sizes can beabout 50 nm, 100 nm, 500 nm, 1 μm, 10 μm, 80 μm, or 120 μm. The smallerthe sizes of the apertures 105, the less dislocation defects will occurduring the growth of the first semiconductor layer 120. In oneembodiment, the sizes of the apertures 105 are in a range from about 10nm to about 10 μm. The duty factor of the carbon nanotube layer 102 isan area ratio between the sheltered epitaxial growth surface 101 and theexposed epitaxial growth surface 101. The duty factor of the carbonnanotube layer 102 can be in a range from about 1:100 to about 100:1,such as about, 1:10, 1:2, 1:4, 4:1, 2:1, or 10:1. In one embodiment, theduty factor of the carbon nanotube layer 102 is in a range from about1:4 to about 4:1.

In one embodiment, the carbon nanotubes in the carbon nanotube layer 102are arranged to extend along a direction substantially parallel to thesurface of the carbon nanotube layer 102 to obtain a better pattern andgreater light transmission. After being placed on the epitaxial growthsurface 101, the carbon nanotubes in the carbon nanotube layer 102 arearranged to extend along the direction substantially parallel to theepitaxial growth surface 101. Referring to FIG. 2, all the carbonnanotubes in the carbon nanotube layer 102 are arranged to extendsubstantially along the same direction. Referring to FIG. 4, part of thecarbon nanotubes in the carbon nanotube layer 102 are arranged to extendsubstantially along a first direction. The other part of the carbonnanotubes in the carbon nanotube layer 102 are arranged to extend alonga second direction substantially perpendicular to the second direction.Also the carbon nanotubes in the ordered carbon nanotube structure canbe arranged to extend substantially along the crystallographicorientation of the substrate 100 or along a direction which forms anangle with the crystallographic orientation of the substrate 100.

The carbon nanotube layer 102 can be formed on the epitaxial growthsurface 101 by chemical vapor deposition (CVD), transfer printing apreformed carbon nanotube film, filtering, or depositing a carbonnanotube suspension. In one embodiment, the carbon nanotube layer 102 isa free-standing structure and can be drawn from a carbon nanotube array.The term “free-standing structure” means that the carbon nanotube layer102 can sustain the weight of itself if it is hoisted by a portionthereof without any significant damage to its structural integrity.Thus, the carbon nanotube layer 102 can be suspended by two spacedsupports. The free-standing carbon nanotube layer 102 can be laid on theepitaxial growth surface 101 directly and easily.

The carbon nanotube layer 102 can be a continuous structure or adiscontinuous structure. The discontinuous carbon nanotube layer 102includes a number of carbon nanotube wires substantially parallel toeach other. If the carbon nanotube layer 102 has carbon nanotube wiressubstantially parallel to each other and a supporting force is appliedto the carbon nanotube layer 102 in a direction substantiallyperpendicular to axial directions of the carbon nanotube wires, theparallel carbon nanotube wires can form a free-standing structure. Thesuccessive carbon nanotubes are joined end to end by van der Waalsattractive force in a direction substantially parallel to an axialdirection of the carbon nanotube and the carbon nanotubes are connectedwith each other by van der Waals attractive force in a directionsubstantially perpendicular to an axial direction of the carbonnanotubes.

The carbon nanotube layer 102 can be a substantially pure structure ofthe carbon nanotubes, with few impurities and chemical functionalgroups. The carbon nanotube layer 102 can be a composite including acarbon nanotube matrix and some non-carbon nanotube materials. Thenon-carbon nanotube materials can be graphite, graphene, siliconcarbide, boron nitride, silicon nitride, silicon dioxide, diamond, oramorphous carbon, metal carbides, metal oxides, or metal nitrides. Thenon-carbon nanotube materials can be coated on the carbon nanotubes ofthe carbon nanotube layer 102 or filled in the apertures 105. In oneembodiment, the non-carbon nanotube materials are coated on the carbonnanotubes of the carbon nanotube layer 102 so the carbon nanotubes canhave greater diameter, and the apertures 105 can have smaller sizes. Thenon-carbon nanotube materials can be deposited on the carbon nanotubesof the carbon nanotube layer 102 by CVD or physical vapor deposition(PVD), for example sputtering.

Furthermore, the carbon nanotube layer 102 can be treated with anorganic solvent after being placed on the epitaxial growth surface 101so the carbon nanotube layer 102 can be attached on the epitaxial growthsurface 101 firmly. Specifically, the organic solvent can be applied toentire surface of the carbon nanotube layer 102 or the entire carbonnanotube layer 102 can be immerged in an organic solvent. The organicsolvent can be volatile, for example ethanol, methanol, acetone,dichloroethane, chloroform, or mixtures thereof. In one embodiment, theorganic solvent is ethanol.

The carbon nanotube layer 102 can include at least one carbon nanotubefilm, at least one carbon nanotube wire, or a combination thereof. Inone embodiment, the carbon nanotube layer 102 can include a singlecarbon nanotube film or two or more stacked carbon nanotube films. Thus,the thickness of the carbon nanotube layer 102 can be controlled by thenumber of stacked carbon nanotube films. The number of stacked carbonnanotube films can be in a range from about 2 to about 100, for exampleabout, 10, 30 or 50. In one embodiment, the carbon nanotube layer 102can include a layer of substantially parallel and spaced carbon nanotubewires. Also, the carbon nanotube layer 102 can include a plurality ofcarbon nanotube wires crossed, or weaved together to form a carbonnanotube net. The distance between two adjacent parallel and spacedcarbon nanotube wires can be in a range from about 0.1 μm to about 200μm. In one embodiment, the distance between two adjacent parallel andspaced carbon nanotube wires can be in a range from about 10 μm to about100 μm. The size of the apertures 105 can be controlled by the distancebetween two adjacent parallel and spaced carbon nanotube wires. Thelength of the gap between two adjacent parallel carbon nanotube wirescan be equal to the length of the carbon nanotube wire. It is understoodthat any carbon nanotube structure described can be used with allembodiments.

A drawn carbon nanotube film is composed of a plurality of carbonnanotubes. A large majority of the carbon nanotubes in the drawn carbonnanotube film can be oriented along a preferred orientation, meaningthat a large majority of the carbon nanotubes in the drawn carbonnanotube film are arranged substantially along the same direction. Anend of one carbon nanotube is joined to another end of an adjacentcarbon nanotube arranged substantially along the same direction by vander Waals attractive force. The drawn carbon nanotube film is capable offorming a freestanding structure. The successive carbon nanotubes joinedend to end by van der Waals attractive force realizes the freestandingstructure of the drawn carbon nanotube film.

Some variations can occur in the orientation of the carbon nanotubes inthe drawn carbon nanotube film. Microscopically, the carbon nanotubesoriented substantially along the same direction may not be perfectlyaligned in a straight line, and some curve portions may exist. A contactbetween some carbon nanotubes located substantially side by side andoriented along the same direction cannot be totally excluded.

The structure of the drawn carbon nanotube film and the method formaking the drawn carbon nanotube film is illustrated as follows.

Referring to FIGS. 2 and 3, each drawn carbon nanotube film includes aplurality of successively oriented carbon nanotube segments 143 joinedend-to-end by van der Waals attractive force therebetween. Each drawncarbon nanotube segment 143 includes a plurality of carbon nanotubes 145substantially parallel to each other, and combined by van der Waalsattractive force therebetween. The drawn carbon nanotube segments 143can vary in width, thickness, uniformity, and shape. The carbonnanotubes in the drawn carbon nanotube film are also substantiallyoriented along a preferred orientation. A thickness of the drawn carbonnanotube film can range from about 1 nm to about 100 μm in oneembodiment. The thickness of the drawn carbon nanotube film can rangefrom about 100 nm to about 10 μm in another embodiment. A width of thedrawn carbon nanotube film relates to the carbon nanotube array fromwhich the drawn carbon nanotube film is drawn. The apertures between thecarbon nanotubes in the drawn carbon nanotube film can form theapertures 105 in the carbon nanotube layer 102. The apertures betweenthe carbon nanotubes in the drawn carbon nanotube film can be less than10 μm. Examples of the drawn carbon nanotube film are taught by U.S.Pat. No. 7,045,130 to Jiang et al., and WO 2007015710 to Zhang et al.

The carbon nanotube layer 102 includes at least two drawn carbonnanotube films stacked with each other. In other embodiments, the carbonnanotube layer 102 can include two or more coplanar carbon nanotubefilms, and can include layers of coplanar carbon nanotube films.Additionally, if the carbon nanotubes in the carbon nanotube film arealigned along one preferred orientation (e.g., the drawn carbon nanotubefilm), an angle can exist between the orientation of carbon nanotubes inadjacent films, whether stacked or adjacent. Adjacent carbon nanotubefilms can be combined by van der Waals attractive force therebetween. Anangle between the aligned directions of the carbon nanotubes in the twoadjacent drawn carbon nanotube films can range from about 0 degrees toabout 90 degrees (0°≦α≦90°). If α is about 0°, the two adjacent drawncarbon nanotube films are arranged in the same direction with eachother. If the angle between the aligned directions of the carbonnanotubes in adjacent stacked drawn carbon nanotube films is larger than0 degrees, a plurality of micropores is defined by the carbon nanotubelayer 102. Referring to FIG. 6, the carbon nanotube layer 102 shown withthe angle between the aligned directions of the carbon nanotubes inadjacent stacked drawn carbon nanotube films is about 90 degrees. Thestacked drawn carbon nanotube films can improve the strength andmaintain the shape of the carbon nanotube layer 102. Stacking the carbonnanotube films will also increase the structural integrity of the carbonnanotube layer 102.

Furthermore, the carbon nanotube layer 102 can be heated to decrease thethickness of the carbon nanotube layer 102. If the carbon nanotube layer102 is heated, the carbon nanotubes with larger diameter will absorbmore energy and be destroyed. The carbon nanotube layer 102 can beheated locally to protect the carbon nanotube layer 102 from damage. Inone embodiment, the carbon nanotube layer 102 is heated by dividing asurface of the carbon nanotube layer 102 into a number of local areasand heating all of the local areas of the carbon nanotube layer 102 oneby one. The carbon nanotube layer 102 can be heated by a laser or amicrowave. In one embodiment, the carbon nanotube layer 102 is heated bythe laser and a power density of the laser is greater than 0.1×102 W/m².

The laser can irradiates the carbon nanotube layer 102 in many ways. Thedirection of the laser can be substantially perpendicular to the surfaceof the carbon nanotube layer 102. The moving direction of the laser canbe substantially parallel or perpendicular to axial directions of thecarbon nanotubes in the carbon nanotube layer 102. For a laser with astable power density and wavelength, the slower the moving speed of thelaser, the more the carbon nanotubes of the carbon nanotube layer 102will be destroyed, and the thinner the carbon nanotube layer 102.However, if the speed is too slow, the carbon nanotube layer 102 will becompletely destroyed. In the present embodiment, a power density of thelaser is about 0.053×10¹² W/m², a diameter of the irradiating pattern ofthe laser is in a range from about 1 mm to about 5 mm, and a time oflaser irradiation is less than 1.8 seconds. In the present embodiment,the laser is a carbon dioxide laser and the power density of the laseris about 30 W, a wavelength of the laser is about 10.6 μm, and thediameter of the irradiating pattern of the laser is about 3 mm. A movingspeed of the laser device is less than 10 m/s.

The carbon nanotube wire can be an untwisted carbon nanotube wire or atwisted carbon nanotube wire. Both of the untwisted carbon nanotube wireor twisted carbon nanotube wire can be a free-standing structure.Referring to FIG. 5, the untwisted carbon nanotube wire includes aplurality of carbon nanotubes substantially oriented along a directionalong the length of the untwisted carbon nanotube wire. Specifically,the untwisted carbon nanotube wire includes a plurality of successivecarbon nanotube treated segment joined end to end by van der Waalsattractive force therebetween. Each carbon nanotube treated segmentincludes a plurality of carbon nanotubes substantially parallel to eachother, and combined by van der Waals attractive force therebetween. Thecarbon nanotube treated segments can vary in width, thickness,uniformity, and shape. The length of the untwisted carbon nanotube wirecan be arbitrarily set as desired. A diameter of the untwisted carbonnanotube wire is in a range from about 0.5 nm to about 100 μm. Theuntwisted carbon nanotube wire is formed by treating the carbon nanotubefilm with an organic solvent. Specifically, the carbon nanotube film istreated by applying the organic solvent to the carbon nanotube film tosoak the entire surface of the carbon nanotube film. After being soakedby the organic solvent, adjacent paralleled carbon nanotubes in thecarbon nanotube film will bundle together due to the surface tension ofthe organic solvent as the organic solvent volatilizes, and thus, thecarbon nanotube film will be shrunk into an untwisted carbon nanotubewire.

The twisted carbon nanotube wire is formed by twisting a carbon nanotubefilm by a mechanical force to turn the two ends of the carbon nanotubefilm in opposite directions. Referring to FIG. 6, the twisted carbonnanotube wire includes a plurality of carbon nanotubes oriented aroundan axial direction of the twisted carbon nanotube wire. The carbonnanotubes are aligned around the axis of the carbon nanotube twistedwire like a helix. More specifically, the twisted carbon nanotube wireincludes a plurality of successive carbon nanotube segment joined end toend by van der Waals attractive force therebetween. Each carbon nanotubesegment includes a plurality of carbon nanotubes substantially parallelto each other, and combined by van der Waals attractive forcetherebetween. The carbon nanotube segments can vary in width, thickness,uniformity, and shape. The length of the carbon nanotube wire can bearbitrarily set as desired. A diameter of the twisted carbon nanotubewire can be in a range from about 0.5 nm to about 100 μm.

Further, the twisted carbon nanotube wire can be treated with a volatileorganic solvent. After being soaked by the organic solvent, the adjacentparalleled carbon nanotubes in the twisted carbon nanotube wire willbundle together as the organic solvent volatilizes. The specific surfacearea of the twisted carbon nanotube wire will decrease, and the densityand strength of the twisted carbon nanotube wire will increase. Examplesof the carbon nanotube wire are taught by U.S. Pat. No. 7,045,130 toJiang et al., and US 20100173037 A1 to Jiang et al.

As discussed above, the carbon nanotube layer 102 can be used as a maskfor growing the first semiconductor layer 120. The term ‘mask’ meansthat the carbon nanotube layer 101 can be used to shelter part of theepitaxial growth surface 101 and expose the other part of the epitaxialgrowth surface 101. Thus, the first semiconductor layer 120 can growfrom the exposed epitaxial growth surface 101. The carbon nanotube layer101 can form a pattern mask on the epitaxial growth surface 101 becausethe carbon nanotube layer 102 defines a plurality of first openings 105.The method of forming a carbon nanotube layer 102 as mask is simple, lowcost, and will not pollute the substrate 100 when compared tolithography or etching.

The carbon nanotube layer 102 can be suspended in any manner as long asthe suspended carbon nanotube layer 102 corresponding to the epitaxialgrowth surface 101 of the substrate 100 is spaced from and suspendedabove the epitaxial growth surface 101 of the substrate 100. In oneembodiment, two opposite ends of the carbon nanotube layer 102 are fixedand pulled up to suspend the carbon nanotube layer 102. In anotherembodiment, two opposite ends of the carbon nanotube layer 102 arefastened on two spaced supporters to suspend the carbon nanotube layer102. In one embodiment, the carbon nanotube layer 102 is substantiallyparallel to and spaced from the epitaxial growth surface 101 of thesubstrate 100. The extending directions of the carbon nanotubes in thecarbon nanotube layer 102 are substantially parallel to the epitaxialgrowth surface 101. The distance between the carbon nanotube layer 102and the epitaxial growth surface 101 can be in a range from about 10 nmto about 500 μm. In one embodiment, the distance between the carbonnanotube layer 102 and the epitaxial growth surface 101 can be in arange from about 50 nm to about 500 μm. In another embodiment, thedistance between the carbon nanotube layer 102 and the epitaxial growthsurface 101 is about 10 μm. The carbon nanotube layer 102 is very closeto the epitaxial growth surface 101, therefore the first semiconductorlayer 120 can permeate the carbon nanotube layer 102 and enclose thecarbon nanotube layer 102 easily during the growth of the firstsemiconductor layer 120. The dislocation density in the firstsemiconductor layer 120 is also low. As a result, the efficiency ofmaking the LED 10 is improved and the cost of making the LED is low.

In one embodiment, a method for suspending the carbon nanotube layer 102includes:

S121, providing a supporting device;

S122, fixing the carbon nanotube layer 102 on the supporting device; and

S123, suspending the carbon nanotube layer 102 above the epitaxialgrowth surface 101 by the supporting device.

In step S121, the material of the supporting device should have acertain mechanical strength to support the carbon nanotube layer 102. Inone embodiment, the material of the supporting device may be pure metaland metal alloy or conductive composites. In one embodiment, thesupporting device includes a first support 112 and a second support 114spaced from the first support 112. A distance between the first support112 and the second support 114 is selected according to a size of thesubstrate 100. In one embodiment, the distance between the first support112 and the second support 114 is larger than the size of the substrate100. Therefore, the entire carbon nanotube layer 102 is suspended. Theshape of the first support 112 and the second support 114 should have aplant surface and the end of the carbon nanotube layer 102 can befastened to the plant surface. In one embodiment, each of the firstsupport 112 and the second support 114 is a cuboid. In anotherembodiment, the supporting device is a frame. The shape of the frame isthe same as the substrate 100 and the size of the frame is larger thanthe substrate 100. The edge of the carbon nanotube layer is fastened onthe frame.

In step S122, one end of the carbon nanotube layer 102 is fastened onthe first support 112, and an opposite end of the carbon nanotube layer102 is fastened on the second support 114. The carbon nanotube layer 102has a certain viscosity, therefore, the carbon nanotube layer 102 can befastened to the supporting device directly. The carbon nanotube layer102 is suspended and stretched by the supporting device.

In step S123, the first support 112 and the second support 114 arelocated on opposite sides of the substrate 100, respectively.

In step S13, the first semiconductor layer 120, the active layer 130,and the second semiconductor layer 140 are grown in sequence by amolecular beam epitaxy (MBE), chemical beam epitaxy (CBE), vacuumepitaxy, low temperature epitaxy, selective epitaxy, liquid phasedeposition epitaxy (LPE), metal organic vapor phase epitaxy (MOVPE),ultra-high vacuum chemical vapor deposition (UHVCVD), hydride vaporphase epitaxy (HYPE), or metal organic chemical vapor deposition(MOCVD).

Materials of the first semiconductor layer 120, the active layer 130,and the second semiconductor layer 140 can be identical. The materialsof the first semiconductor layer 120, the active layer 130, and thesecond semiconductor layer 140 can be controlled by changing thematerial of the source gas during the growth procession.

A thickness of the first semiconductor layer 120 can be selectedaccording to need. The thickness of the first semiconductor layer 120can be in a range from about 200 nm to about 15 μm. In one embodiment,the thickness of the first semiconductor layer 120 may be about, 300 nm,500 nm, 1 μm, 3 μm, 5 μm, or 10 μm. The first semiconductor layer 120can be an N-type semiconductor layer or a P-type semiconductor layer.The N-type semiconductor layer provides electrons, and the P-typesemiconductor layer provides cavities. The N-type semiconductor layercan be made of N-type gallium nitride, N-type gallium arsenide, orN-type copper phosphate. The P-type semiconductor layer can be made ofP-type gallium nitride, P-type gallium arsenide, or P-type copperphosphate. In one embodiment, the first semiconductor layer 120 is aSi-doped N-type gallium nitride semiconductor layer.

The active layer 130 is a photon exciting layer and can be one of asingle quantum well layer or multilayer quantum well films. The activelayer 130 can be made of gallium indium nitride (GaInN), aluminum indiumgallium nitride (AlGaInN), gallium arsenide (GaSn), aluminum galliumarsenide (AlGaSn), gallium indium phosphide (GaInP), or aluminum galliumarsenide (GaInSn). The active layer 130, in which the cavities thereinare filled by the electrons, can have a thickness of about 0.01 μm toabout 0.6 μm. In one embodiment, the active layer 130 has a thickness ofabout 0.3 μm and includes a layer of InGaN/GaN.

The second semiconductor layer 140 can be an N-type semiconductor layeror a P-type semiconductor layer. The type of the second semiconductorlayer 140 is different from the type of the first semiconductor layer120. If the first semiconductor layer 120 is an N-type semiconductor,the second semiconductor layer 140 is a P-type semiconductor, and viceversa. A thickness of the second semiconductor layer 140 is in a rangefrom about 0.1 μm to about 3 μm. A surface of the second semiconductorlayer 140 away from the substrate 100 can act as a light-emitting faceof the LED 10. In one embodiment, the second semiconductor layer 140 canbe an Mg-doped P-type gallium nitride semiconductor layer, and athickness of the second semiconductor layer 140 is about 0.3 μm.

In one embodiment, the first semiconductor layer 120 is prepared by theMOCVD method. During the MOCVD method, H₂, N₂ or a mixture thereof canbe used as a carrier gas, trimethyl gallium can be used as a Ga source,silane can be used as a silicon source, and ammonia can be used as anitrogen source gas. The MOCVD method for making the first semiconductorlayer 120 comprises the following steps:

S131, putting the substrate 100 with the carbon nanotube layer 102thereon into a reaction chamber, flowing a carrier gas into the reactionchamber, and heating the reaction chamber to about 1100° C. to about1200° C. for about 200 sec to about 1000 sec;

S132, growing a low-temperature GaN layer by cooling the reactionchamber to about 500° C. to about 650° C. and flowing trimethyl galliumand ammonia gas into the reaction chamber;

S133, stopping the flow of the trimethyl gallium, increasing thetemperature of the reaction chamber to about 1100° C. to about 1200° C.,and then maintaining the temperature of the reaction chamber constantfor about 30 sec to 300 sec;

S134, growing an intrinsic semiconductor layer by maintaining thetemperature of the reaction chamber in a range from about 1000° C. toabout 1100° C. and the pressure in the reaction chamber at about 100torr to about 300 torr, and flowing the trimethyl gallium into thereaction chamber; and

S135, growing a doped semiconductor layer by maintaining the temperatureof the reaction chamber at about 1000° C. to about 1100° C., and furtherflowing silane into the reaction chamber.

In step S131, the substrate 100 is sapphire.

In step S132, referring to FIG. 14, the low-temperature GaN layer can beused as a buffer layer 1201. A thickness of the low-temperature GaNlayer can be in a range from about 10 nm to about 50 nm. Thelow-temperature GaN layer can reduce the lattice mismatch between thefirst semiconductor layer 120 and the sapphire substrate 100. Therefore,the dislocation density of the first semiconductor layer 120 is low. Thematerial of the buffer layer 1201 can also be aluminium nitride.

In step S134, a thickness of the intrinsic semiconductor layer 1202 canbe in a range from about 100 nm to about 10 μm.

The buffer layer 1201, the intrinsic semiconductor layer 1202 and thedoped semiconductor layers 1203 are defined as the first semiconductorlayer 120. In the above-described MOCVD method, the trimethyl galliumcan be substituted with triethyl gallium.

During the growth of the first semiconductor layer 120, if the firstsemiconductor layer 120 contacts the carbon nanotube layer 102, thefirst semiconductor layer 120 grows through the apertures 105 of thecarbon nanotube layer 102. The first semiconductor layer 120 then growsfrom flanks of the carbon nanotubes in the carbon nanotube layer 102.The first semiconductor layer 120 further grows along a directionsubstantially parallel to the epitaxial growth surface 101 and enclosesthe carbon nanotubes of the carbon nanotube layer 102. Therefore, anumber of channels 103 are formed in the first semiconductor layer 120for the carbon nanotube layer 102.

In one embodiment, a number of epitaxial grains of the firstsemiconductor layer 120 grow from the substrate 100. If the epitaxialgrains contact the carbon nanotube layer 102, the epitaxial grains growthrough the apertures 105 of the carbon nanotube layer 102 and furthergrow along a direction substantially parallel to the epitaxial growthsurface 101 to connect together and enclose the carbon nanotube layer102. The epitaxial grains further grow along a direction substantiallyperpendicular to the epitaxial growth surface 101 to form the firstsemiconductor layer 120. A number of channels 103 are formed in thefirst semiconductor layer 120 due to the carbon nanotubes. At least onesingle carbon nanotube or one carbon nanotube bundle is located in eachchannel 103.

The cross-section of the channel 103 can be geometrically shaped. Adiameter of the channel 103 is in a range from about 20 nm to about 200nm. In one embodiment, the diameter of the channel 103 is in a rangefrom about 50 nm to about 100 nm. The channels 103 form a patternedmicrostructure in the first semiconductor layer 120. The patternedmicrostructure in the first semiconductor layer 120 corresponds to thepattern of the carbon nanotube layer 102. If the carbon nanotube layer102 includes a number of cross-stacked carbon nanotube films or a numberof carbon nanotube wires crossed with each other or woven together, thechannels 103 in the first semiconductor layer 120 form an interconnectedchannel network. The carbon nanotubes in the channel network constitutethe conductive carbon nanotube layer 102. If the carbon nanotube layer102 includes a number of carbon nanotube wires substantially parallel toand spaced from each other or a drawn carbon nanotube film, the channels103 in the first semiconductor layer 120 are substantially parallel toand spaced from each other. In one embodiment, distances between twoadjacent channels 103 are substantially equal.

A method for growing the active layer 130 is similar to the method forgrowing the first semiconductor layer 120. The active layer 130 is grownafter the step of growing the first semiconductor layer 120. During thegrowth of the active layer 130, the trimethyl indium is used as theindium source. In one embodiment, a method for growing the active layer130 includes the following steps:

Step a1, stopping the flow of the silane into the reaction chamber afterthe step S135 of growing the first semiconductor layer 120, heating thereaction chamber to about 700° C. to about 900° C., and maintainingpressure of the reaction chamber at about 50 torr to about 500 torr; and

Step a2, forming the active layer 130 by flowing trimethyl indium intothe reaction chamber to grow InGaN/GaN multi-quantum well layer.

A method for growing the second semiconductor layer 140 is similar tothe method for growing the first semiconductor layer 120. The secondsemiconductor layer 140 is grown after growing the active layer 130.During the growth of the second semiconductor layer 140, ferrocenemagnesium can be used as the magnesium source. In one embodiment, themethod for growing the second semiconductor layer 140 includes thefollowing steps:

Step b1, stopping the flow of the trimethyl indium into the reactionchamber after the step a2 of growing the active layer 130, heating thereaction chamber to about 1000° C. to about 1100° C. and maintaining thepressure of the reaction chamber at about 76 torr to about 200 torr; and

Step b2, forming the second semiconductor layer 140 by flowing ferrocenemagnesium into the reaction chamber to grow Mg-doped P-type GaN layer.

In step S14, the portion of the second semiconductor layer 140 and theactive layer 130 are etched by a reactive ion etching. After the activelayer 130 is etched, a portion of the first semiconductor layer 120 canalso be etched by reactive ion etching. However, after the firstsemiconductor layer 120 is etched, the carbon nanotube layer 102 shouldnot be exposed. The substrate 100, the carbon nanotube layer 102, thefirst semiconductor layer 120, the active layer 130, and the secondsemiconductor layer 140 constitute an LED chip.

In one embodiment, the active layer 130 is made of InGaN/GaN layer andthe second semiconductor layer 140 is made of P-type GaN layer. Thesecond semiconductor layer 140 and the active layer 130 can be etched byplacing the LED chip into an inductively coupled plasma device andadding a mixture of silicon tetrachloride and chlorine into theinductively coupled plasma device. In one embodiment, the power of theinductively coupled plasma device is about 50 W, the speed of thechlorine is about 26 sccm, and the speed of the silicon tetrachloride isabout 4 sccm. The partial pressure of the silicon tetrachloride andchlorine is about 2 Pa. The etched thickness of the second semiconductorlayer 140 is about 0.3 μm. The etched thickness of the active layer 130is about 0.3 μm.

In step S15, the first electrode 150 is located on the exposed surfaceof the first semiconductor layer 120, and the second electrode 160 islocated on a top surface of the second semiconductor layer 140. Thefirst electrode 150 may be a P-type or an N-type electrode and is thesame type as the first semiconductor layer 120. The second electrode 160may be a P-type or an N-type electrode and is the same type as thesecond semiconductor layer 140.

A thickness of the first electrode 150 can range from about 0.01 μm toabout 2 μm. A thickness of the second electrode 160 can range from about0.01 μm to about 2 μm. The first electrode 150 can be made of titanium,aluminum, nickel, gold, or a combination thereof. In one embodiment, thefirst electrode 150 is an N-type electrode and includes a nickel layerand a gold layer. A thickness of the nickel layer is about 15 nm. Athickness of the gold layer is about 100 nm. In one embodiment, thesecond electrode 160 is a P-type electrode and includes a titanium layerand a gold layer. A thickness of the titanium layer is about 15 nm. Athickness of the gold layer is about 100 nm.

The first electrode 150 and the second electrode 160 can be formedsimultaneously. The first electrode 150 and the second electrode 160 canbe formed by PVD, for example, electron beam evaporation, vacuumevaporation, and ion sputtering method.

The method for making the LED 10 described above has many benefits. Onebenefit is the carbon nanotube layer 102 is a free-standing structure.Therefore, the carbon nanotube layer 102 can be laid directly on thesubstrate 100 directly without difficulty. Another benefit is thechannels 103 are formed between the first semiconductor layer 120 andthe substrate 100 without etching to avoid damage to the latticestructure of the LED. Yet another benefit is the carbon nanotubes in thecarbon nanotube layer 102 are small enough so that the size of thegrains of the first semiconductor layer 120 around the carbon nanotubesis small and dislocations of the first semiconductor layer 120 arefewer.

Referring to FIG. 7, an LED 10 is illustrated in one embodiment. The LED10 includes a substrate 100, a carbon nanotube layer 102, a firstsemiconductor layer 120, an active layer 130, a second semiconductorlayer 140, a first electrode 150, and a second electrode 160. The firstsemiconductor layer 120, the active layer 130, and the secondsemiconductor layer 140 are stacked on one side of the substrate 100 inthat order. The second semiconductor layer 140 away from the substrate100 can be used as a light emitting surface. The first semiconductorlayer 120, the active layer 130, and the second semiconductor layer 140form a ladder-shaped structure. The first semiconductor layer 120 isoriented to the substrate 100. The carbon nanotube layer 102 is locatedin the interior of the first semiconductor layer 120. The firstelectrode 150 is electrically connected to the first semiconductor layer120. The second electrode 160 is electrically connected to the secondsemiconductor layer 140. A number of channels 103 are formed in theinterior of the first semiconductor layer 120. The carbon nanotube layer102 is located in the channels 103 of the first semiconductor layer 120.At least one carbon nanotube is located in each of the channels 103.

The carbon nanotube layer 102 is a free-standing structure. The carbonnanotube layer 102 includes at least one carbon nanotube film or anumber of carbon nanotube wires. The carbon nanotube layer 102 defines anumber of apertures 105. The apertures 105 extend through the carbonnanotube layer 102 along a thickness direction of the carbon nanotubelayer 102. The aperture 105 can be a hole defined by several adjacentcarbon nanotubes or a gap defined by two substantially parallel carbonnanotubes and extending along axial directions of the carbon nanotubes.

The first semiconductor layer 120 penetrates the apertures 105 of thecarbon nanotube layer 102 and encloses the carbon nanotube layer 102therein. The first semiconductor layer 120 is a continuous structure.The term “continuous structure” means that the first semiconductor layer120 is not broken. A portion of the carbon nanotubes contacts the innerwall of the channels 103. If the carbon nanotube layer 102 includes thedrawn carbon nanotube layer or the carbon nanotube wires aresubstantially parallel to and spaced from each other, the channels 103are a plurality of strip channels 103 substantially paralleled to andspaced apart from each other. If the carbon nanotube layer 102 includesthe carbon nanotube wires crossed with each other or a number ofcross-stacked carbon nanotube films, the channels 103 form a channelnetwork and the channel network is interconnected. If the carbonnanotube layer 104 is composed of a number of cross-stacked carbonnanotube film, angles defined between the carbon nanotubes in twoadjacent carbon nanotube films is greater than 0 degrees and less than90 degrees. The channels 103 form a microstructure in the firstsemiconductor layer 120. The cross-section of the channels 103 can begeometrically shaped. In one embodiment, the shape of the cross-sectionof the channels 103 is substantially round with a diameter in a rangefrom about 2 nm to about 200 μm.

The LED 10 described-above has many benefits. One benefit is a number ofchannels 103 exit in the interior of the first semiconductor layer 120,the channels 103 can change the directions of lights emitted from theactive layer 130, and the large angle lights can be emitted out of theLED 10. Therefore the light extracting rate of the LED 10 can beimproved. Another benefit is the carbon nanotube layer 102 has goodthermal conductivity. The heat produced in the LED 10 can be conductedout of the LED 10 by the carbon nanotube layer 102, thereby prolongingthe life span of the LED 10.

Referring to FIG. 8, an LED 20 is illustrated in one embodiment. The LED20 includes a substrate 100, a carbon nanotube layer 102, a firstsemiconductor layer 120, an active layer 130, a second semiconductorlayer 140, a first electrode 150, and a second electrode 160. The firstsemiconductor layer 120, the active layer 130, and the secondsemiconductor layer 140 are stacked on one side of the substrate 100 inthat order. The second semiconductor layer 140 away from the substrate100 can be used as a light emitting surface. The first semiconductorlayer 120, the active layer 130, and the second semiconductor layer 140constitute a ladder-shaped structure. The first semiconductor layer 120is oriented to the substrate 100. The carbon nanotube layer 102 islocated in the first semiconductor layer 120. The first electrode 150 iselectrically connected to the first semiconductor layer 120. The secondelectrode 160 is electrically connected to the second semiconductorlayer 140.

The structure of the LED 20 is similar to the structure of the LED 10.The difference is that in the LED 20, a portion of the carbon nanotubelayer 102 is exposed and the first electrode 150 is electricallyconnected to the carbon nanotube layer 102. The remaining portion of thecarbon nanotube layer 102 is enclosed in the first semiconductor layer120, the second electrode 160 is transparent and covers the entiresurface of the second semiconductor layer 120, and the thickness of thesecond electrode 160 is thin.

A method for making the LED 20 of one embodiment is similar to themethod for making the LED 10. The difference is that in step S14, aftera portion of the second semiconductor layer 140 and the active layer 130is etched, a portion of the first semiconductor layer 120 is furtheretched to expose a portion of the carbon nanotube layer 102. In stepS15, the first electrode 150 is formed on the surface of the exposedcarbon nanotube layer 102, the second electrode 160 covers the entiresurface of the second semiconductor layer 140, and the second electrode160 is transparent.

Referring to FIG. 9, a method for making an LED 30 of one embodimentincludes the following steps of:

S21, providing a substrate 100 having an epitaxial growth surface 101;

S22, suspending a carbon nanotube layer 102 above the epitaxial growthsurface 101; and

S23, growing a first semiconductor layer 120, an active layer 130 and asecond semiconductor layer 140 on the epitaxial growth surface 101 inthat order, wherein the first semiconductor layer 120 includes a bufferlayer, an intrinsic semiconductor layer, and a doped semiconductorlayer, the carbon nanotube layer 102 is enclosed in the dopedsemiconductor layer to form a microstructure in the first semiconductorlayer 120;

S24, removing the substrate 100, the buffer layer, and the intrinsicsemiconductor layer to expose the doped semiconductor layer; and

S25, preparing a first electrode 150 electrically connected to the firstsemiconductor layer 120 and preparing a second electrode 160electrically connected to the doped semiconductor layer of the secondsemiconductor layer 140.

The steps S21, S22, S23 in the method for making the LED 30 is the sameas the step S11, S12, S13 in the method for making the LED 10.

In the step S24, the substrate 100 can be removed by laser irradiation,etching, or thermal expansion and contraction, depending on the materialof the substrate 100 and the first semiconductor layer 120. In oneembodiment, the substrate 100 is removed by laser irradiation. Thesubstrate 100 can be removed from the first semiconductor layer 120 bythe following steps:

S241, polishing and cleaning the surface of the substrate 100 far awayfrom the first semiconductor layer 120;

S242, locating the substrate 100 on a platform (not shown) andirradiating the substrate 100 and the first semiconductor layer 120 by alaser; and

S243, immersing the substrate 100 into a solvent and removing thesubstrate 100.

In step S241, the substrate 100 can be polished by a mechanicalpolishing method or a chemical polishing method to obtain a smoothsurface. Thus the scattering of laser will be reduced. The substrate 100can be cleaned with hydrochloric acid or sulfuric acid to remove themetallic impurities and oil.

In step S242, the substrate 100 is irradiated by the laser from thepolished surface, and the incidence angle of the laser is substantiallyperpendicular to the surface of the substrate 100. The wavelength of thelaser is selected according to the material of the first semiconductorlayer 120 and the substrate 100. The energy of the laser is smaller thanthe band-gap energy of the substrate 100 and larger than the bandgapenergy of the first semiconductor layer 120. Thus the laser can passthrough the substrate 100 and reach the interface between the substrate100 and the first semiconductor layer 120. The buffer layer oriented tothe substrate 100 has a strong absorption of the laser, and thetemperature of the buffer layer will be raised rapidly. Thus the bufferlayer will be decomposed. In one embodiment, the band-gap energy of thefirst semiconductor layer 120 is about 3.3 ev, and the band-gap energyof the substrate 100 is about 9.9 ev. The laser is a KrF laser, thewavelength of the laser is about 248 nm, and the energy is about 5 ev,the pulse width range about 20 nanosecond to about 40 nanosecond, theenergy density ranges from about 400 mJ/cm² to about 600 mJ/cm², and theshape of the laser pattern is square with a size of 0.5 mm×0.5 mm. Thelaser moves from one edge of the substrate 100 with a speed of about 0.5mm/s During the irradiating process, the GaN is decomposed to Ga and N₂.The parameter of the laser can be adjusted according to need. Thewavelength of the laser can be selected according to the absorption ofthe buffer layer.

Because the buffer layer has a strong absorption of the laser, thebuffer layer decomposes rapidly. But the first semiconductor layer 120has a weak absorption, so it can not be decomposed so readily. Theirradiating process can be performed in a vacuum or a protective gasenvironment to prevent the oxidation of the carbon nanotubes. Theprotective gas can be nitrogen, helium, argon, or other inert gas.

In step S243, the substrate 100 can be immersed into an acidic solutionto remove the Ga decomposed from GaN, so that the substrate 100 can bepeeled off from the first semiconductor layer 120. The acidic solutioncan be hydrochloric acid, sulphuric acid, nitric acid, or any other acidto dissolve the Ga.

Furthermore, the intrinsic semiconductor layer can also be decomposed byion etching or wet etching. After the intrinsic semiconductor layer isremoved, the doped semiconductor layer is exposed. In one embodiment, aplasma etching method includes providing a inductively coupled plasmadevice, flowing a mixture of silicon tetrachloride, and adding chlorineto the inductively coupled plasma device to etch the intrinsicsemiconductor layer. In one embodiment, the power of the inductivelycoupled plasma device is about 50 W, the speed of the chlorine is about26 sccm, the speed of the silicon tetrachloride is about 4 sccm, and thepartial pressure of the silicon tetrachloride and chlorine is about 2Pa.

The thermal expansion and contraction method means that while thesubstrate 100 is heated to a high temperature above 1000° C. and cooledto a low temperature below 1000° C. in a short time from 2 minutes toabout 20 minutes. The substrate 100 separates from the firstsemiconductor layer 120 by cracking because of the thermal expansionmismatch between the substrate 100 and the first semiconductor layer120.

The method for making the first electrode 150 is the same as the methodfor making the second electrode 160. The first electrode 150 and thesecond electrode 160 can be made via a process of physical vapordeposition, for example electron beam evaporation, vacuum evaporation,ion sputtering, physical deposition, or the like. A conductive layer canbe laid on the surface of the doped semiconductor layer directly to formthe first electrode 150. The first electrode 150 is electricallyconnected to the first semiconductor layer 120. The first electrode 150covers the entire surface of the first semiconductor layer 120 in oneembodiment.

Referring to FIG. 10, an LED 30 is illustrated in one embodiment. TheLED 30 includes a substrate 100, a carbon nanotube layer 102, a firstsemiconductor layer 120, an active layer 130, a second semiconductorlayer 140, a first electrode 150, and a second electrode 160. The firstsemiconductor layer 120, the active layer 130, and the secondsemiconductor layer 140 are stacked on one side of the substrate 100 inthat order. The second semiconductor layer 140 away from the substrate100 can be used as a light emitting surface. The first semiconductorlayer 120 is oriented to the substrate 100. The carbon nanotube layer102 is located in the interior of the first semiconductor layer 120. Thefirst semiconductor layer 120 is a doped semiconductor layer. In oneembodiment, the first electrode 150 covers the entire surface of thefirst semiconductor layer 120 and the second electrode 160 covers theentire surface of the second semiconductor layer 140.

Referring to FIG. 11, a method for making an LED 40 of one embodimentincludes the following steps of:

S31, providing a substrate 100 having an epitaxial growth surface 101;

S32, suspending a first carbon nanotube layer 102 above the epitaxialgrowth surface 101;

S33, growing a first semiconductor layer 120, an active layer 130, and asecond semiconductor layer 140 on the epitaxial growth surface 101 inthat order, wherein the first carbon nanotube layer 102 is enclosed inthe first semiconductor layer 120 to form a microstructure in the firstsemiconductor layer 120;

S34, forming a third semiconductor layer 170 on a surface of the secondsemiconductor layer 140, wherein the third semiconductor layer 170includes a number of spaced protrusions to make the third semiconductorlayer 170 discontinuous;

S35, exposing a portion of the first semiconductor layer 120 by etchinga portion of the third semiconductor layer 170, the second semiconductorlayer 140, and the active layer 108; and

S36, preparing a first electrode 150 on the first semiconductor layer120 and preparing a second electrode 160 on the second semiconductorlayer 140.

The method for making the LED 40 is similar to the method for making theLED 10. The difference is that the method for making the LED 40 furthercomprises the step S34 of forming a third semiconductor layer 170 on thesurface of the second semiconductor layer 140.

In step S34, the third semiconductor layer 170 can be formed byphotolithography or imprinting.

A method for forming the third semiconductor layer 170 in one embodimentincludes the following steps:

S341, placing a second carbon nanotube layer in the surface of thesecond semiconductor layer 140, wherein the second carbon nanotube layerdefines a plurality of apertures;

S342, epitaxially growing the third semiconductor layer 170 from thesurface of the second semiconductor layer 140; and

S343, removing the second carbon nanotube layer.

In step S341, the structure and material of the second carbon nanotubelayer are the same as the first carbon nanotube layer 102. The secondcarbon nanotube layer can be used as a mask for growing the thirdsemiconductor layer 170. The term ‘mask’ means that the second carbonnanotube layer can be used to shelter part of the second semiconductorlayer 140 and expose the other part of the second semiconductor layer140. Thus, the third semiconductor layer 170 can grow from the exposedsurface of the second semiconductor layer 140. The second carbonnanotube layer can form a pattern mask on the second semiconductor layer140 because the second carbon nanotube layer defines a plurality ofapertures.

The carbon nanotubes in the second carbon nanotube layer aresubstantially parallel to the surface of the second semiconductor layer140. The second carbon nanotube layer includes a number of carbonnanotubes. The extending directions of the axial directions of thecarbon nanotubes in the second carbon nanotube layer can besubstantially parallel or cross with the extending directions of thecarbon nanotubes in the first carbon nanotube layer 102. In oneembodiment, axial directions of the carbon nanotubes of the secondcarbon nanotube layer are oriented substantially along one direction. Inother embodiments, the second carbon nanotube layer includes a number ofcross-stacked carbon nanotube films or a number of carbon nanotube wirescrossed with each other or woven together to form a network structure.

In step S342, a number of epitaxial crystal grains are nucleated andgrown from the surface of the second semiconductor layer 140 along adirection substantially perpendicular to the surface of the secondsemiconductor layer 140. The epitaxial crystal grains grow from theexposed part of the second semiconductor layer 140 and through theapertures of the second carbon nanotube layer. The epitaxial crystalgrains form a number of discontinuous protrusions 172 on the surface ofthe second semiconductor layer 140. The protrusions 172 form the thirdsemiconductor layer 170. The protrusions 172 are spaced to form a numberof openings between two adjacent protrusions 172. The second carbonnanotube layer is located in the openings. In one embodiment, theopenings are strip-shaped. The extending directions of the strip-shapedopenings are substantially parallel to the surface of the secondsemiconductor layer 140. A width of the strip-shaped openings is in arange from about 20 nm to about 200 nm. In one embodiment, the width ofthe strip-shaped openings is in a range from about 50 nm to about 100nm. The thickness of the third semiconductor layer 170 can be controlledby the growth time of the epitaxial crystal grains. In one embodiment,the thickness of the third semiconductor layer 170 is about 2 μm.

If axial directions of the carbon nanotubes of the second carbonnanotube layer are oriented along one direction, the protrusions 172 arestrip-shaped and the strip-shaped protrusions 172 are spaced from andsubstantially parallel to each other. The extending directions of thestrip-shaped protrusions 172 can be substantially parallel to or crosswith the extending directions of the channels 103 in the firstsemiconductor layer 120. In one embodiment, a width of the strip-shapedprotrusions 172 can be in a range from about 20 nm to about 200 nm. Inanother embodiment, the width of the strip-shaped protrusions 172 can bein a range from about 50 nm to about 100 nm. If the second carbonnanotube layer includes a number of cross-stacked carbon nanotube filmsor a number of carbon nanotube wires crossed with each other or woventogether to form a network structure, the protrusions 172 can bescattered dot-shaped protrusions 172. The dot-shaped protrusions 172 areuniformly located on the surface of the second semiconductor layer 140.A size of the dot-shaped protrusions can be in a range from about 10 nmto about 10 μm.

The material of the third semiconductor layer 170 can be GaN, GaAs orCuP. The material of the third semiconductor layer 170 can be the sameas or different from the second semiconductor layer 140. In oneembodiment, the material of the third semiconductor layer 170 isMg-doped GaN.

In step S343, the second carbon nanotube layer can be removed by plasmaetching method, laser heating method, ultrasonic wave method, or furnaceheating method. In one embodiment, the second carbon nanotube layer isremoved by the laser heating method. The laser heating method includesthe following steps:

S3432, providing a laser generator which can generate a laser toirradiate the second carbon nanotube layer; and

S3434, scanning the second carbon nanotube layer by making a relativemovement between the laser generator and the second carbon nanotubelayer in an environment containing oxygen gas.

In step S3432, the laser generator can be a solid laser generator,liquid laser generator, gas laser generator, and semiconductor lasergenerator. A power density of the laser is larger than 0.053×10¹² W/m².As the laser irradiates the second carbon nanotube layer, a laser beamproduced by the laser device is focused on the second carbon nanotubelayer and forms a laser irradiating area, e.g., a circular area, on thesecond carbon nanotube layer, wherein a diameter of the laserirradiating area can be in a range from about 1 mm to about 5 mm. Thelaser beam is substantially perpendicular to the surface of the secondcarbon nanotube layer. An irradiating time of the laser can be shorterthan about 1.8 sec. In one embodiment, the laser generator is a carbondioxide laser generator, a power of the carbon dioxide laser generatoris about 30 W, a wavelength of the laser is about 10.6 μm, and adiameter of the laser irradiating area is about 3 mm.

In step S3434, the direction of movement of the laser beam can besubstantially parallel to or perpendicular to the axial directions ofthe carbon nanotubes in the second carbon nanotube layer. The carbonnanotubes can absorb the energy of the laser and be heated by the laser,and the carbon nanotubes can then react with the oxygen gas and beremoved. The reaction time can be controlled by adjusting the relativemoving speed between the laser generator and the second carbon nanotubelayer. If the power density and the wavelength of the laser is fixed,the slower the relative moving speed of the laser generator and thesecond carbon nanotube layer, the longer the irradiation time of thecarbon nanotubes. The longer the irradiation time of the carbonnanotubes, the more energy the carbon nanotubes absorb, and the easierthe carbon nanotubes oxidizes. In one embodiment, the relative movingspeed between the laser generator and the carbon nanotube layer 104 isless than 10 mm/sec.

It is to be understood that in step S342, when the thickness of theepitaxial crystal grains is larger than the thickness of the secondcarbon nanotube layer, the epitaxial crystal grains can further growalong a direction substantially parallel to the second semiconductorlayer 140 and substantially enclose the entire second carbon nanotubelayer. The epitaxial crystal grains then form a continuous thirdsemiconductor layer 170. The step of removing the second carbon nanotubelayer can then be omitted.

The epitaxial growth method for making the third semiconductor layer 170by locating the second carbon nanotube layer as a mask is simple, low incost, and readily available when compared to the traditionalnano-imprinting method or etching method. The steps S35 and S36 in themethod for making the LED 40 can be substituted by step S24 and step S25in the method for making the LED 30.

Referring to FIG. 12, an LED 40 is illustrated in one embodiment. TheLED 40 includes a substrate 100, a carbon nanotube layer 102, a firstsemiconductor layer 120, an active layer 130, a second semiconductorlayer 140, a first electrode 150, and a second electrode 160. The firstsemiconductor layer 120, the active layer 130, and the secondsemiconductor layer 140 are stacked on one side of the substrate 100 inthat order. The second semiconductor layer 140 away from the substrate100 can be used as a light emitting surface. The first semiconductorlayer 120 is oriented to the substrate 100. The carbon nanotube layer102 is located in the interior of the first semiconductor layer 120 toform a number of channels 103 in the first semiconductor layer 120. Thecarbon nanotube layer 102 is located in the channels 103. At least onecarbon nanotube is located in each of the channels 103.

The structure of the LED 40 is similar to the structure of the LED 10.The difference is that a third semiconductor layer 170 is located on thesurface of the second semiconductor layer 140, the third semiconductorlayer 170 includes a number of protrusions 172 spaced from each other toform a number of openings. The third semiconductor layer 170 isdiscontinuous. In one embodiment, the protrusions 172 can be a number ofstrip-shaped protrusions 172. The strip-shaped protrusions 172 arespaced from and substantially parallel to each other. The cross-sectionof the strip-shaped protrusions 172 can be geometrically shaped. In oneembodiment, the size of the cross-section of the strip-shapedprotrusions 172 can be in a range from about 10 nm to about 100 nm. Inanother embodiment, a size of the cross-section of the strip-shapedprotrusions 172 is in a range from about 20 nm to about 50 nm. A widthof the strip-shaped protrusions 172 is in a range from about 10 nm toabout 10 μm. In one embodiment, the strip-shaped protrusions 172 form aprotrusion network. The protrusions 172 are interconnected.

The third semiconductor layer 170 forms a microstructure located on thesurface of the second semiconductor layer 140. If large angle lightsemitted from the active layer 130 travel to the third semiconductorlayer 170, directions of the large angle lights will change and thelarge angle lights can pass through the third semiconductor layer 170without being internally reflected. Therefore, the light extracting rateof the LED 40 will be improved.

Referring to FIG. 13, an LED 50 is illustrated in one embodiment. TheLED 50 includes a substrate 100, a first carbon nanotube layer 102, asecond carbon nanotube layer 104, a first semiconductor layer 120, anactive layer 130, a second semiconductor layer 140, a thirdsemiconductor layer 170, a first electrode 150, and a second electrode160. The first semiconductor layer 120, the active layer 130, and thesecond semiconductor layer 140 are stacked on one side of the substrate100 in that order. The first semiconductor layer 120 is oriented to thesubstrate 100. The second electrode 160 can be used as a light emittingsurface. A portion of the first carbon nanotube layer 102 is enclosed inthe first semiconductor layer 120. A remaining portion of the firstcarbon nanotube layer 102 is exposed. The first electrode 150 iselectrically connected to the exposed portion of the first carbonnanotube layer 102. The first electrode 150 is electrically connected tothe first semiconductor layer 120 by the first carbon nanotube layer102. The third semiconductor layer 170 includes a number of spacedprotrusions 172. The third semiconductor layer 170 is discontinuous. Thesecond electrode 160 is transparent and covers the entire exposedsurface of the second semiconductor layer 140. Furthermore, the secondelectrode 160 covers the entire surface of the third semiconductor layer170 and the second carbon nanotube layer 104. The second electrode 160is transparent and very thin. The second carbon nanotube layer 104 islocated between the second electrode 160 and the second semiconductorlayer 140. The protrusions 172 are spaced from each other to define anumber of openings. The second carbon nanotube layer 104 is located inthe openings formed by two adjacent protrusions 172. In one embodiment,a number of gaps are formed between the carbon nanotubes of the secondcarbon nanotube layer 104 and the protrusions 172, the second carbonnanotube layer 104 permeates the gaps between the second carbon nanotubelayer 104 and the protrusions 172 and contacts with the secondsemiconductor layer 140.

The structure of the LED 50 is similar to the structure of the LED 40.The difference is that a portion of the first carbon nanotube layer 102is enclosed in the first semiconductor layer 120, a remaining portion ofthe first carbon nanotube layer 102 is exposed, the second electrode 160is transparent and covers the entire exposed surface of the secondsemiconductor layer 140, the second carbon nanotube layer 104 is locatedin the openings defined by two adjacent protrusions 172, the secondcarbon nanotube layer 104 is located in the openings formed by twoadjacent protrusions 172, and the second electrode 160 is transparentand very thin.

A method for making the LED 50 is similar to the method for making theLED 40. The method for making the LED 50 includes the following steps:

S51, providing a substrate 100 having an epitaxial growth surface;

S52, suspending a first carbon nanotube layer 102 above the epitaxialgrowth surface;

S53, growing a first semiconductor layer 120, an active layer 130 and asecond semiconductor layer 140 on the epitaxial growth surface in thatorder, wherein the first carbon nanotube layer 102 is enclosed in thefirst semiconductor layer 120;

S54, placing a second carbon nanotube layer 104 on the surface of thesecond semiconductor layer 140, wherein the second carbon nanotube layer104 has a number of apertures;

S55, growing a third semiconductor layer 170 on a surface of the secondsemiconductor layer 140, wherein the third semiconductor layer 170 growsthrough the apertures of the second carbon nanotube layer 104;

S56, etching a portion of the third semiconductor layer 170, the secondcarbon nanotube layer 104, the second semiconductor layer 140, theactive layer 108 and a portion of first semiconductor layer 120 toexpose a portion of the first carbon nanotube layer 102; and

S57, preparing a first electrode 114 on the exposed portion of the firstcarbon nanotube layer 102 and preparing a second electrode 112 on thesecond semiconductor layer 140 to cover the entire surface of the thirdsemiconductor layer 170 and the second carbon nanotube layer 104.

The method for making the LED 50 is similar to the method for making theLED 40. The difference is that in step S343 of removing the secondcarbon nanotube layer can be omitted, in step S55, a portion of firstsemiconductor layer 120 is etched to expose a portion of the firstcarbon nanotube layer 102, in step S56, the first electrode 150 isformed on the surface of the exposed first carbon nanotube layer 102,the second electrode 160 covers the entire surface of the thirdsemiconductor layer 170 and the second carbon nanotube layer 104, andthe second electrode 160 is transparent and very thin.

It is to be understood that the above-described embodiments are intendedto illustrate rather than limit the disclosure. Any elements describedin accordance with any embodiments is understood that they can be usedin addition or substituted in other embodiments. Embodiments can also beused together. Variations may be made to the embodiments withoutdeparting from the spirit of the disclosure. The above-describedembodiments illustrate the scope of the disclosure but do not restrictthe scope of the disclosure.

Depending on the embodiment, certain of the steps of methods describedmay be removed, others may be added, and the sequence of steps may bealtered. It is also to be understood that the description and the claimsdrawn to a method may include some indication in reference to certainsteps. However, the indication used is only to be viewed foridentification purposes and not as a suggestion as to an order for thesteps.

What is claimed is:
 1. A method for making a light emitting diode, themethod comprising: providing a substrate having an epitaxial growthsurface; suspending a carbon nanotube layer above the epitaxial growthsurface; growing a first semiconductor layer, an active layer, and asecond semiconductor layer on the epitaxial growth surface in thatorder, wherein the first semiconductor layer comprises a buffer layer,an intrinsic semiconductor layer, and a doped semiconductor layerstacked in that order; exposing the doped semiconductor layer byremoving the substrate, the buffer layer, and the intrinsicsemiconductor layer; and preparing a first electrode electricallyconnected to the first semiconductor layer and a second electrodeelectrically connected to the second semiconductor layer.
 2. The methodof claim 1, wherein the carbon nanotube layer is a free-standingstructure.
 3. The method of claim 1, wherein a distance between thecarbon nanotube layer and the epitaxial growth surface is in a rangefrom about 10 nanometers to about 500 micrometers.
 4. The method ofclaim 1, wherein suspending the carbon nanotube layer comprises thefollowing steps: providing a supporting device; fixing the carbonnanotube layer on the supporting device; and suspending the carbonnanotube layer above the epitaxial growth surface by the supportingdevice.
 5. The method of claim 1, wherein the carbon nanotube layercovers the entire epitaxial growth surface.
 6. The method of claim 1,wherein the carbon nanotube layer defines a plurality of apertures. 7.The method of claim 1, wherein the carbon nanotube layer comprises acarbon nanotube film or a plurality of carbon nanotube wiressubstantially parallel to and spaced from each other.
 8. The method ofclaim 7, wherein a plurality of channels is formed in the dopedsemiconductor layer, the channels are substantially parallel to andspaced from each other, and the carbon nanotube layer is located in thechannels.
 9. The method of claim 1, wherein the carbon nanotube layercomprises a plurality of stacked carbon nanotube films and a pluralityof carbon nanotube wires crossed with each other or woven together toform a carbon nanotube network.
 10. The method of claim 9, wherein achannel-network is formed in the doped semiconductor layer, and thecarbon nanotube layer is located in the channel-network.
 11. The methodof claim 6, wherein the doped semiconductor layer grows through theapertures of the carbon nanotube layer, and along a directionsubstantially parallel to the epitaxial growth surface to connecttogether and enclose the carbon nanotube layer.
 12. The method of claim11, wherein the doped semiconductor layer further grows after the dopedsemiconductor layer is enclosing the carbon nanotube layer.
 13. Themethod of claim 1, wherein removing the substrate comprises thefollowing steps: polishing and cleaning the surface of the substrate faraway from the first semiconductor layer; locating the substrate on aplatform and irradiating the substrate and the first semiconductor layerby a laser; and immersing the substrate into a solvent and removing thesubstrate.
 14. The method of claim 1, wherein the buffer layer isremoved in the process of irradiating the substrate by the laser. 15.The method of claim 1, wherein the intrinsic semiconductor layer isremoved by etching.
 16. The method of claim 1, wherein a portion of thefirst semiconductor layer is etched after a portion of the secondsemiconductor layer and the active layer is etched, so that a portion ofthe carbon nanotube layer is exposed, and the first electrode covers theexposed surface of the carbon nanotube layer.
 17. The method of claim 1,wherein the second electrode covers the entire surface of the secondsemiconductor layer.